The present invention is related to transmission control mechanisms and, more particularly, to electro-hydraulic control mechanisms for multi-speed transmissions.
Multi-speed power transmissions, particularly those using planetary gear arrangements, require a hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the clutches and brakes which operate to establish the ratios within the planetary gear arrangement.
These control systems have evolved from substantially pure hydraulic control systems, wherein all of the control signals are produced by hydraulic devices, to electro-hydraulic control systems, wherein a number of the control signals are produced by an electronic control unit which emits electrical control signals to solenoid valves which then issue controlled hydraulic signals to the various operating valves within the transmission control.
With many of the early pure hydraulic and first generation electro-hydraulic control systems, the power transmission utilized a number of freewheel or one-way devices which smooth the shifting or ratio interchange of the transmission during both upshifting and downshifting of the transmission. This relieved the hydraulic control system from providing for the control of overlapping between the friction device that was coming on and the friction device that was going off. If this overlap is too much, the driver feels a shudder in the drivetrain, and if the overlap is too little, the driver experiences engine flare or a sense of coasting. The freewheel device prevents this feeling that, as is well known, the freewheel one-way devices will quickly engage when the torque imposed thereon is reversed from a freewheeling state to a transmitting state.
The advent of electro-hydraulic devices gave rise to what is known as clutch-to-clutch shift arrangements to reduce the complexity of the transmission and the control. These electro-hydraulic control mechanisms are generally perceived to reduce cost and reduce the space required for the control mechanism.
Also, with the advent of more sophisticated control mechanisms, the power transmissions have advanced from two-speed or three-speed transmissions to five-speed and six-speed transmissions. In at least one presently available six-speed transmission, just five friction devices are employed to provide six forward speeds and neutral condition and a reverse speed. Such a gear arrangement is shown in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978.
The use of the planetary gearset shown in the Polak patent has given rise to a number of electro-hydraulic control mechanisms, such as that shown in U.S. Pat. No. 5,601,506, issued to Long et al. on Feb. 11, 1997. This control utilizes a solenoid trim pressure system that is limited in flexibility in that the transmission control can only provide single step speed ratio interchanges. Two other electro-hydraulic control mechanisms shown in U.S. Pat. Nos. 4,827,806 and 5,616,093, both issued to the assignee of this application, provide drive-home capabilities within the transmission in the event that the electronic system undergoes a malfunction or discontinuance of operation.
The drive-home feature of a power transmission is an important factor in that it permits the vehicle operator to return home with the vehicle so that the proper repairs can be undertaken at a repair station rather than in the field where the vehicle underwent the malfunction.
It is an object of this invention to provide an improved electro-hydraulic control mechanism for a multi-speed power transmission.
In one aspect of the present invention, an electro-hydraulic control mechanism permits improved operation of the transmission in the event of an electrical malfunction. In another aspect of the present invention, the improved electro-hydraulic control permits the operator to select a reverse drive, a neutral condition, and a forward drive in the event of an electrical power cessation. In yet another aspect of the present invention, the improved electro-hydraulic control will default to a forward high-speed range in the event of an electrical power discontinuance when the transmission is operating at or above a predetermined high-speed range.
In yet a further aspect of the present invention, the improved electro-hydraulic control will permit the selection of a lower forward speed range, following a manual shift from reverse to forward, during discontinuance of electrical power in the high speed operation. In yet still another aspect of the present invention, the improved electro-hydraulic control mechanism will permit operation in a lower speed range in the event that a malfunction occurs below the above-mentioned predetermined speed range.
In yet a further aspect of the present invention, the improved electro-hydraulic control mechanism includes a default valve, a multiplex valve, a manual valve, and a single on/off solenoid control to establish the forward/neutral/reverse operation if an electrical shutdown occurs. In yet an even further aspect of the present invention, the multiplex valve of the improved electro-hydraulic control mechanism prevents simultaneous engagement of mutually exclusive torque transmitting devices or friction devices.